To aid in measuring the radiation passing through a patient, detector channels of CT equipment detect the X-rays and convert them to electrical signals. One type of such transducers uses scintillating material. Scintillating material is material that emits light photons responsive to impingment thereon by X-rays. An example of scintillating material is cadmium tungstate (CDWO4) crystals. However, this invention is not limited to scintillating crystals, but will operate just as well with other transducers that suffer from afterglow problems.
The scintillating crystals serve as transducers of X-ray radiation to transform the X-ray radiation into optical radiation. When X-ray photons are absorbed by a scintillator, photo-electrons are ejected from the atoms of the scintillator. This photo-electron loses its energy through multiple interactions with other electrons. The secondary electrons release part of their energy in the form of optical radiation and the crystal "scintillates".
The crystal contains what are known as electron traps. When an electron is trapped, its scintillation will be delayed and an afterglow will occur. Aftergow is the existence of scintillation after the cessation of the X-ray radiation. The origin of the afterglow is not clear, it may be caused by such things as chemical impurities or crystal structure imperfections. It is known that a crystal afterglow scintillation decays exponentially when irradiated by a step function. The afterglow, therfore, seems to have a "memory" effect and acts as a low-pass filter.
In the past, to overcome the afterglow, materials which supposedly do not provide afterglow have been used. However, it has been found that most of the other materials that generate less afterglow than cadmium tungsten crystals, also provide an initial "glow" having a lower intensity or amplitude than that of the crystal scintillators which do suffer from afterglow.
Another attempt at overcoming the afterglow problem used presently is the preselection of the detectors in an effort to use only detectors that have no afterglow. This method, of course, is extremely expensive since a relatively large proportion of the detectors have to be discarded because of afterglow problems.
Thus the prior art methods of overcoming or compensating for the afterglow artifacts has been to use materials providing lower intensities and/or selecting detectors that have less afterglow than the average detector. The prior art correction methods are either expensive in money and time and/or expensive in the intensity of the final signals.
Accordingly, those skilled in the art are continuously searching for methods and means for overcoming the afterglow caused artifacts.